The dialects of Middle English
The dialectal position of Middle English is basically a continuation of that of Old English. The most important extralinguistic fact for the development of the Middle English dialects is that the capital of the country was moved from Winchester (in the Old English period) to London by William the Conqueror in his attempt to diminish the political influence of the native English.
NORTHERN This dialect is the continuation of the Northumbrian variant of Old English. Note that by Middle English times English had spread to (Lowland) Scotland and indeed led to a certain literary tradition developing there at the end of the Middle English period which has been continued up to the present time (with certain breaks, admittedly).
Characteristics. Velar stops are retained (i.e. not palatalised) as can be seen in word pairs like rigg/ridge; kirk/church.
KENTISH This is the most direct continuation of an Old English dialect and has more or less the same geographical distribution.
Characteristics. The two most notable features of Kentish are (1) the existence of /e:/ for Middle English /i:/ and (2) so-called "initial softening" which caused fricatives in word-initial position to be pronounced voiced as in vat, vane and vixen (female fox).
SOUTHERN West Saxon is the forerunner of this dialect of Middle English. Note that the area covered in the Middle English period is greater than in the Old English period as inroads were made into Celtic-speaking Cornwall. This area becomes linguistically uninteresting in the Middle English period. It shares some features of both Kentish and West Midland dialects.
WEST MIDLAND This is the most conservative of the dialect areas in the Middle English period and is fairly well-documented in literary works. It is the western half of the Old English dialect area Mercia.
Characteristics. The retention of the Old English rounded vowels /y:/ and /ø:/ which in the East had been unrounded to /i:/ and /e:/ respectively.
EAST MIDLAND This is the dialect out of which the later standard developed. To be precise the standard arose out of the London dialect of the late Middle English period. Note that the London dialect naturally developed into what is called Cockney today while the standard became less and less characteristic of a certain area and finally (after the 19th century) became the sociolect which is termed Received Pronunciation.
Characteristics. In general those of the late embryonic Middle English standard.